1. Field of the Invention
The present invention relates to an antenna used in, for example, an RFID (Radio Frequency Identification) system that communicates with an external apparatus via an electromagnetic field signal, and to a mobile terminal including the antenna.
2. Description of the Related Art
Japanese Patent No. 3975918 discloses an antenna to be mounted in a portable electronic apparatus for use in an RFID system. The antenna of Japanese Patent No. 3975918 includes an asymmetrical coil in which the winding interval is different between two portions opposing each other across the center of a coil winding region. A magnetic body is provided on a side opposite a principal surface opposing an IC card in a portion where the winding interval is larger, and the magnetic body is provided on the principal surface opposing the IC card in a portion where the winding interval is smaller.
However, when the electronic apparatus including the antenna disclosed in Japanese Patent No. 3975918 is held over a reader/writer, communication may be disabled according to the angle formed between the antenna of the electronic apparatus and an antenna of the reader/writer. That is, there are few problems when communication is performed in a state in which the IC card and the mobile terminal are parallel to each other. However, when communication is performed with a built-in reader/writer instead of the IC card, the position or angle between the mobile terminal and the reader/writer is not fixed, and this deteriorates communication performance.
FIGS. 1A and 1B include views illustrating magnetic fluxes that interlink with a coil of an antenna similar to the antenna of Japanese Patent No. 3975918 when an angle formed between the antenna and an antenna of a reader/writer is changed. In FIGS. 1A and 1B, both solid arrows and broken arrows schematically indicate magnetic fluxes.
A first principal surface of a magnetic sheet 1 faces toward a side close to a planar conductor 2, a first conductor portion 11 of a coil conductor is close to the first principal surface of the magnetic sheet 1, and a second conductor portion 12 of the coil conductor is close to a second principal surface of the magnetic sheet 1. An antenna coil formed by the magnetic sheet 1 and the coil conductor is located near an end of the planar conductor 2 so as to construct an antenna.
When the magnetic sheet 1 is parallel to a reader/writer side antenna 30, as illustrated in FIG. 1A, that is, when the angle formed therebetween is 0°, a magnetic flux, shown by the solid line, entering the magnetic sheet 1 from the right side is stronger than a magnetic flux, shown by the broken line, entering from the left side, and electromotive force is generated in the coil.
However, when the angle between the magnetic sheet 1 and the reader/writer side antenna 30 is increased from that state, the magnetic flux entering from a side far from the reader/writer (e.g., right side) decreases, and the magnetic flux entering from a side close to the reader/writer (e.g., left side) increases. Hence, at a certain angle, a point where the right and left magnetic fluxes become completely equal and no electromotive force is generated occurs. In this state, communication is impossible.